Key term
AQA A-Level Computer Science 7517
Fundamentals of Programming
Topic overviewData typesProgramming concepts and program structureArithmetic operationsRelational Operations, Constants and VariablesBoolean operationsString-handling operationsRandom numbers and exception handlingSubroutines, parameters and return valuesVariable scope and stack framesRecursive techniquesProgramming paradigms and the procedural approachClasses, Objects, Inheritance and PolymorphismAggregation, Composition, Class Diagrams and Design Principles
Fundamentals of Data Structures
Fundamentals of Algorithms
Theory of Computation
Topic overviewLogic Problems and Algorithm ConstructsPseudo-Code, Hand-Tracing and Program CorrectnessRepresentational Abstraction, Generalisation and Information HidingProcedural and Functional AbstractionData Abstraction and Problem ReductionDecomposition and CompositionAutomation and Finite State MachinesSets and Set NotationSubsets and Set OperationsRegular expressions and regular languagesBackus-Naur Form and syntax diagramsComparing Algorithms and FunctionsOrder of complexity and Big-O notationLimits of computation, tractability and heuristicsComputable problems and the Halting problemTuring machines
Fundamentals of Data Representation
Topic overviewNumber systems and the sets of numbersNumber bases: decimal, binary and hexadecimalBits, bytes and units of informationUnsigned binary and binary arithmeticSigned binary using two's complementFixed point and floating point representationNormalisation of floating point numbersRounding errors, accuracy, range and precisionCharacter coding systemsError checking and correctionBit patterns, analogue and digitalBitmapped graphicsVector graphicsDigital representation of soundMIDI and Data CompressionEncryption
Fundamentals of Computer Systems
Topic overviewHardware, Software and Its ClassificationSystem Software and the Operating SystemTranslators and the Role of the Operating SystemClassification of programming languagesTypes of program translatorLogic Gates and Their Truth TablesLogic Circuits and D-Type Flip-FlopsHalf-Adders and Full AddersBoolean algebra
Fundamentals of Computer Organisation and Architecture
Topic overviewInternal components, buses and von Neumann and Harvard architecturesThe Stored Program Concept and the ProcessorThe Fetch-Execute cycleThe processor instruction setAddressing modesMachine-Code OperationsShifts, Halting and Assembly Language ProgramsInterruptsFactors affecting processor performanceInput and output devicesSecondary storage devices
Fundamentals of Communication and Networking
Topic overviewCommunication methodsCommunication basicsNetwork Topology and Types of NetworkingWireless networkingThe Internet and Packet SwitchingRouting, URLs and Domain NamesFirewalls, Worms, Trojans and VirusesEncryption, Digital Signatures and CertificatesTCP/IPStandard application layer protocolsIP Address Structure and Subnet MaskingIP Standards and Public and Private AddressesDHCP and Network Address TranslationClient server modelThin- versus thick-client computing
Consequences of Uses of Computing
Systematic Approach to Problem Solving
Fundamentals of Databases
Big Data
Fundamentals of Communication and Networking
TCP/IP
- 2 things to know
- Paper 2
Overview
Recall questions on the four layers and what each does, which addresses change at each hop, sockets and port numbers, and why both MAC and IP addresses are needed.
What you need to know
Answer each one before you reveal it.
Your recallNot started
Process
Why does a device need a MAC address as well as an IP address?
Questions from this subtopic
Each one with the model answer underneath.
- 1
Name the four layers of the TCP/IP stack in order from the top down.
Model answer
Application, transport, network, link.
- 2
Describe the role of the application layer.
Model answer
The application layer is the top layer, closest to the user. It provides the protocols that applications use to communicate in a way both ends understand — for example the protocols for fetching web pages, sending and retrieving email, transferring files and logging in remotely. It specifies the format and structure of the messages the application sends, and passes that data down to the transport layer. It has no interest in how the data actually gets there.
- 3
Describe the role of the transport layer.
Model answer
The transport layer establishes an end-to-end connection between the two hosts and makes the transfer reliable. It splits the data from the application layer into numbered packets, adds the source and destination port numbers so the data reaches the right application at each end, and keeps track of which packets have been acknowledged. If a packet is not acknowledged it is resent. At the receiving end this layer uses the sequence numbers to reassemble the packets into the right order and passes the complete data up to the application layer.
- 4
Describe the role of the network layer.
Model answer
The network layer, also called the Internet layer, is responsible for getting packets across the interconnected networks to the correct host. It adds the source and destination IP addresses to each packet, and it routes packets: routers operate at this layer, reading the destination IP address and forwarding the packet towards it. The IP addresses stay the same from one end of the journey to the other.
- 5
Describe the role of the link layer.
Model answer
The link layer is the bottom layer and handles the actual transmission of data over the physical connection between one device and the next. It adds the MAC address of the sending network interface and the MAC address of the device the packet is being handed to on this hop, and it deals with the hardware and the transmission medium itself — cable, radio or fibre.
- 6
As a packet travels across the Internet, which addresses change at each hop and which stay the same? Why?
Model answer
The IP addresses in the network layer stay the same for the entire journey, because they identify the two hosts the conversation is actually between. The MAC addresses in the link layer are replaced at every hop, because they identify only the two pieces of hardware at either end of the single physical link currently being crossed. Each router strips off the incoming link-layer addressing, decides the next hop from the unchanged destination IP address, and writes new MAC addresses for that next step.
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